The present invention relates to a sensor of the position of a moving member over a trajectory followed by this moving member.
There is known, particularly from DE-A-29 23 644 or EP-A-798 541, such a sensor comprising first and second members movable relative to each other, one of the members being secured to the movable member and the other fixed. The first member defines a magnetic circuit having one or several air gaps of which at least one is provided with a sensor delivering a signal representative of the magnetic induction prevailing in this air gap. Such a sensor can be constituted for example by a Hall effect sensor or by an electrical resistance sensitive to induction, as is well known.
The second member comprises a permanent magnet in coupled relation with the magnetic circuit of the first member, so as to induce by its poles a magnetic flow in this circuit. The intensity of the magnetic flow induced is a function of the relative position of the members, and hence of the position of the movable member over its trajectory. The sensor disposed in the air gap delivers a signal proportional to this intensity to electronic means which derive a measurement from this position.
The known sensors of this type are subject to the sensitivity of the sensor used. When the latter is a Hall effect sensor, in particular, it functions satisfactorily only if the permanent magnet is capable of introducing the sensor into a region in which the magnetic induction is of the order of 10 mT, at least. It is necessary for this purpose to use magnets made of costly material.
In certain industries, such as the automotive industry, this is not compatible with the objectives of controlled cost production which most often obtain in mass produced goods for the public at large.
The present invention thus has for its object to provide a sensor of the type described above, operating satisfactorily with permanent magnets of low cost, such as ferrite magnets, for example.
This object of the invention is achieved, as well as others which will become apparent from a reading of the description which follows, with a magnetic sensor of the position of a movable member over a path followed by the movable member, comprising first and second members movable relative to each other, one of the members being secured to said movable member and the other fixed, said first member defining a circuit of ferromagnetic material having at least one air gap provided with a sensor delivering a signal representative of the magnetic induction prevailing in said air gap, said second member comprising a permanent magnet in magnetic coupling relation with said first member to close said magnetic circuit, the sensor being characterized in that the first piece comprises a tubular portion and a portion in the form of a rod extending along the axis of said tubular portion, said portions being magnetically coupled, at one end, by said second member of which said permanent magnet has a radial supply and, at the other end, by means for concentrating magnetic flux in said air gap, this latter being delimited by said concentration means at the end facing the portion in the form of a rod of said first member.
As will be seen later in greater detail, the mentioned flux concentration permits causing the sensor that is used to function correctly, particularly when the latter is of the Hall effect type, and this although this sensor incorporates a permanent magnet whose production cost is low, such as a ferrite magnet for example.
According to other characteristics, which are optional, of the sensor according to the invention:
the concentration means have, in a section through a plane perpendicular to the axis of the first member, a surface which decreases as the section approaches the air gap,
the surface of the terminal section of said concentration means is greater than that of the sensitive surface of the sensor,
the sensor is a Hall effect sensor,
the permanent magnet is of elongated shape, complementary to that of the tubular portion of the first member, so as to be able to enter the latter, the portion in the form of a rod of this member passing through an axial hole of the magnet,
the permanent magnet has the shape of an annular ring pierced by a central hole, the ring being associated with first and second tubular magnetic flux guides, coaxial to the ring and fixed to this latter at the periphery of the hole and of the periphery of the ring respectively, said first and second tubular guides being axially movable with the ring relative to said portions in the form of a rod and a tube, respectively, of said first member.